Mobile cellular communication systems have become of increasing importance, providing mobile users the security of being able to seek aid in case of trouble, allowing dispatching of delivery and other vehicles with little wasted time, and the like. Present cellular communication systems use terrestrial transmitters, such as towers, to define each cell of the system, so that the extent of a particular cellular communication system is limited by the region over which the towers are distributed. Many parts of the world are relatively inaccessible, or, as in the case of the ocean, do not lend themselves to location of a plurality of dispersed cellular sites.
In these regions of the world, spacecraft-based communication systems may be preferable to terrestrial-based systems. It is desirable that a spacecraft cellular communications system adhere, insofar as possible, to the standards which are common to terrestrial systems, and in particular to such systems as the GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS system (GSM), which is in use in Europe.
The GSM system is a cellular communications system which communicates with user terminals by means of electromagnetic transmissions from, and receptions of such electromagnetic signals at, base stations, fixed sites or towers spaced across the countryside. The term "user terminal" for purposes of this patent application includes mobile user terminals, and also includes hand-held and fixed user terminals, but not gateways. The GSM system is described in detail in the text The GSM System for Mobile Communications, subtitled A Comprehensive Overview of the European Digital Cellular System, authored by Michel Mouly and Marie-Bernadette Pautet, and published in 1992 by the authors, at 4, rue Elisee Reclus, F-91120 Palaiseau, France. Another text describing the GSM system is Mobile Radio Communications, by Raymond Steele, published 1992 by Pentech Press, London, ISBN 0-7273-1406-8. Each base station of the GSM system includes transmitter and receiver arrangements, and communicates with user terminals by way of signals in a bandwidth of 50 MHz, centered on 900 Mhz., and also by way of signals having a bandwidth of 150 Mhz centered on 1800 Mhz.
A cellular communication system should provide one or more control channels for allowing a user terminal to initially synchronize to the system, and to initiate communications with the overall network. Each base station, fixed site, or tower continually transmits network synchronization information (SCH) and network-specific information (BCCH), which a user terminal uses to synchronize to the appropriate network at initial turn-on of the user terminal. The GSM system provides a channel denominated "Random Access Channel" or RACH. In GSM, the RACH channel is used for initial synchronization of the network to the user terminal.
Spacecraft cellular communication systems are desired.